uma: Make the cache alignment mask unsigned

In uma_set_align_mask(), ensure that the passed value doesn't have its
highest bit set, which would lead to problems since keg/zone alignment
is internally stored as signed integers. Such big values do not make
sense anyway and indicate some programming error. A future commit will
introduce checks for this case and other ones.

Reviewed by: kib, markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42262